Possible explanation of excess events in the search for jets, missing transverse momentum and a Z boson in pp collisions
aa r X i v : . [ h e p - ph ] J u l LPT Orsay 15-27
Possible explanation of excess events in the search forjets, missing transverse momentum and a Z boson in pp collisions Ulrich Ellwanger aa LPT, UMR 8627, CNRS, Universit´e de Paris–Sud, 91405 Orsay, France, andSchool of Physics and Astronomy, University of Southampton,Highfield, Southampton SO17 1BJ, UK
Abstract
We study to which extent SUSY extensions of the Standard Model can describethe excess of events of 3.0 standard deviations observed by ATLAS in the on- Z signalregion, respecting constraints by CMS on similar signal channels as well as constraintsfrom searches for jets and E miss T . GMSB-like scenarios are typically in conflict withthese constraints, and do not reproduce well the shape of the E miss T distribution ofthe data. An alternative scenario with two massive neutralinos can improve fits to thetotal number of events as well as to the H T and E miss T distributions. Such a scenariocan be realised within the NMSSM. Introduction
After the first run of the LHC at a center of mass (c.m.) energy of mostly 8 TeV, nosignificant excesses have been observed in searches for physics beyond the Standard Model[1,2]. These searches cover a wide range of possible signatures, notably various combinationsof jets, missing transverse energy ( E miss T ), b -jets and leptons (electrons or muons).Same-flavour opposite-sign dileptons can be classified into “off- Z ” leptons (typically withan invariant mass m ll <
81 GeV or m ll >
101 GeV), and “on- Z ” leptons with 81 GeV 101 GeV. Often, leptons and in particular on- Z dileptons are vetoed in order tosuppress Standard Model (SM) backgrounds. On the other hand, some decay cascadesof supersymmetric (SUSY) particles could be particularly rich in off- Z dileptons (in thepresence of light sleptons), or on- Z dileptons if Z bosons appear particularly frequently inthese cascades.Recently, results of searches for SUSY particles in events with dileptons, jets and E miss T have been published by the CMS and ATLAS collaborations [3, 4]. The aim was to testscenarios of gluino pair production in which the gluinos ˜ g decay via sleptons (leading tooff- Z dileptons), and scenarios of gauge mediated SUSY breaking (GMSB) or generalisedgauge mediation (GGM) where the gluinos undergo 3-body decays into quark pairs anda neutralino χ . The latter may decay subsequently into a nearly massless gravitino ˜ G and a Z boson, leading to on- Z dileptons. The corresponding gluino decay chain is then˜ g → q + ¯ q + χ → q + ¯ q + Z + ˜ G . Relevant parameters are the gluino mass m ˜ g , the neutralinomass m χ , and the branching fractions of the involved decays.Whereas no significant excesses were observed by CMS in [3] (up to an excess of 2.6 stan-dard deviations in the dilepton mass window 20 GeV < m ll < 70 GeV), an excess of3.0 standard deviations was reported by ATLAS in [4] in the on- Z signal region: Summingelectron and muon pairs, 29 events passing the cuts were observed versus 10 . ± . m ˜ g − m χ plane of GGM models. Various studies of scenarios which could contribute to this excesshave recently been published [5–13]. Z bosons decay dominantly hadronically. Thus, whenever gluinos are pair produced,in most cases each of the two gluino cascades will produce no dileptons, but two hardjets: either from q + ¯ q if m ˜ g ≫ m χ > ∼ M Z , or from hadronic Z decays if m ˜ g > ∼ m χ ≫ M Z implying a neutralino much heavier than the gravitino, i.e. energetic Z bosons. Hence, bothscenarios are subject to constraints from “standard” searches for SUSY in events with hardjets and E miss T [14, 15], even if one considers simplified models where squarks are assumedto be decoupled and gluino pair production is the only process taken into account.In order to study the impact of these constraints on GMSB-like scenarios, we simulatedvarious configurations of gluino and χ masses. Using the latest version 1.2.0 of CheckMATE[16] we found that constraints from [17] (a preliminary version of [14]) on final states withjets and E miss T are very restrictive, and supersede even the recent CMS constraints from [3]in the m ˜ g − m χ plane. Exceptions are scenarios with reduced branching fractions for theconsidered decay chain, without allowing for alternative final states leading to jets and E miss T .In the present paper we study to which extent a scenario with two heavy neutralinos in1he gluino decay cascade can contribute to the ATLAS signal region, circumventing con-straints from searches for jets and E miss T . The gluino decay cascade considered subsequentlyis of the form ˜ g → q + ¯ q + χ → q + ¯ q + Z + χ (1.1)with m χ < ∼ m ˜ g , m χ ∼ m χ − 100 GeV (1.2)and sketched in Fig. 1. ˜ g ¯ qq ˜ χ Z ˜ χ ˜ g ¯ qq ˜ χ Z ˜ χ Figure 1: Gluino decay cascades involving two neutralinos χ and χ .Now jets from both steps of the gluino decay cascade (including the jets from the Z boson) are relatively soft, and constraints from searches for jets and E miss T are easier tosatisfy unless the mass splitting m ˜ g − m χ is too large. Such a scenario has been consideredrecently also in [9]. We will compare their results to ours in the conclusions.In the following we consider first simplified models with 100% branching fractions forboth steps of the gluino decay cascade. We simulated corresponding events, verified whichscenarios satisfy the constraints from the CMS [3] and other SUSY searches, and appliedthe cuts of ATLAS [4]. We will compare the signal rates and various distributions to thedata given in [4], and to a GMSB-like scenario (the latter with reduced branching fractionsin order to comply with constraints). Constraints from CMS [3] prevent an excess as largeas 3.0 standard deviations in the ATLAS signal region, but about 14 signal events on topof the background are possible.However, the question arises in which SUSY scenario such a neutralino spectrum and,notably, such a dominant gluino decay cascade are possible: What can prevent a dominant˜ g → q + ¯ q + χ decay which is favored by phase space? In GMSB the rˆole of χ is played bythe nearly massless gravitino, which has tiny couplings to the MSSM-like sparticles and isnot produced unless, due to R-parity conservation, it is the only decay channel. A heavierneutralino χ with small couplings to the MSSM-like sparticles, as required in the presentscenario, is possible in the NMSSM [18] in the form of the singlino, the fermionic partnerof the singlet superfield S whose vacuum expectation value generates dynamically a µ -term(a SUSY mass term for the two Higgs doublets in the MSSM) of the order of the SUSY2reaking scale. We find that there exist indeed scenarios within the parameter space of theNMSSM for which the gluino decay cascade in Eq. (1.1) is dominant.In the next section we describe details of the simulation and cuts. Results for simplifiedmodels and the description of a NMSSM scenario are given in section 3. We conclude insection 4. We have simulated events at the LHC at 8 TeV using MadGraph/MadEvent [19] which in-cludes Pythia 6.4 [20] for showering and hadronisation. The emission of one additional hardjet was allowed in the simulation in order to obtain realistic distributions for kinematicalvariables. The production cross sections were obtained by Prospino at NLO [21, 22].First, the output was given to CheckMATE version 1.2.0 [16] which includes the detectorsimulation DELPHES [23] and compares the signal rates to constraints from various searchchannels of ATLAS and CMS. All searches present in CheckMATE version 1.2.0 have beenverified; the most relevant ones (with the largest ratio for the event yield to S where S is the observed 95% CL upper bound) are obtained from the ATLAS search for jets and E miss T in [17].Second, the Pythia output was given directly to DELPHES and analysed according tothe object identification and selection criteria given in [3, 4], respectively, and finally thecorresponding cuts were applied.For the ATLAS on- Z searches [4] these were as follows: E miss T > 225 GeV; ≥ p T > 35 GeV; two same-flavour opposite-sign leptons with p T > 25 GeV for the leading, p T > 10 GeV for the sub-leading lepton; H T > 600 GeV where H T = p lepton,1T + p lepton,2T + P i p jet,iT (including jets with p T > 35 GeV); and finally 81 GeV < m ll < 101 GeV. 29 eventspassing the cuts were observed, whereas 10 . ± . m ll , E miss T , H T and the jet multiplicity N jets were shown in [4] separately for the electron and muon channels. These distributionswere compared with those expected from two GGM benchmark points with gluino massesand neutralino masses of ( m ˜ g , m χ ) = (700 , , E miss T .We compared the expected properties of the two GGM benchmark points in [4] to theresults of our simulation and found that they agree within ∼ ∼ 30% from themore realistic (detector-) simulation of the experimental collaboration. We can expect thatthis systematic error cancels to a large extent when comparing the properties of differentsimulated scenarios, but should be taken into account when comparing to the actual datafrom [4]. Since it is of the same order (actually somewhat larger) than the difference inthe acceptances of dielectrons and dimuons in [4], we found it reasonable to consider thesum of the data of dielectron and dimuon events not only for the signal rate, but also forthe kinematical distributions and the expected SM background in order to obtain a largerstatistics.In the CMS on- Z searches [3], no cuts on H T were applied. Signal jets were required3o have p T > 40 GeV. Six E miss T - and N jets -dependent on- Z signal regions were defined: E miss T = 100 − , − , > 300 GeV and N jets ≥ , ≥ 3, respectively. Finally theCMS and ATLAS analyses differ slightly in the jet algorithms and in the lepton acceptances.Comparing the signal rates obtained by our simulations of the two GMSB-like benchmarkpoints to the simulations in [3] we found again that they agree within ∼ Z signal regionsby CMS. Hence the event yields in the six on- Z signal regions lead to constraints on anyscenarios which attempt to explain the ATLAS excess. In the next section we discussby means of benchmark points to which extent the ATLAS excess can be matched inconsideration of these constraints, as well as constraints from [17] on final states with jetsand E miss T . First we considered GMSB-like simplified models with a branching fraction of 100% for the˜ g → q + ¯ q + χ → q + ¯ q + Z + ˜ G decay chain. Then, however, constraints from the searchfor jets and E miss T in [17] as tested by CheckMATE [16] require m ˜ g > ∼ m χ ∼ 150 GeV, and larger gluino masses for larger m χ . Accordingly contributions to theATLAS signal region cannot exceed ∼ E miss T and H T peak towards large values (most events have H T > m ˜ g , m χ ) = (800 , χ was chosen in order to shift the peak of the H T distribution towards lower values.) Forthe remaining 90% of the gluino decays one has to expect that, depending on the completespectrum and branching fractions, they contribute to the signal regions in the search forjets and E miss T in [17]. One can make the somewhat optimistic assumption that thesecontributions do not exceed 50% of the contributions of the ˜ g → q + ¯ q + χ → q + ¯ q + Z + ˜ G decay chain. Then this point remains within the constraints from [17], but contributesabout 10 events to the ATLAS on-Z signal region.Next we consider simplified models with two heavy neutralinos whose decay chain isdepicted in Fig. 1. Assuming a branching fraction of 100% for this decay chain, gluinos canbe as light as 800 GeV without conflict with constraints from the search for jets and E miss T in[17] – under the condition, however, that m χ and m χ are relatively large such that all jetsremain relatively soft. We studied two benchmark points P1 and P2 with ( m ˜ g , m χ , m χ ) =(800 , , m ˜ g , m χ , m χ ) = (800 , , m ˜ g − m χ = 10 GeV the jets from the first step ˜ g → q + ¯ q + χ of thedecay cascade are very soft, as are the jets from Z decays from the second step χ → Z + χ . Practically all energy of a single gluino decay cascade goes into E miss T . However,for typical kinematical configurations the momenta of χ tend to be back-to-back in the4ransverse plane, leading to a reduction of E miss T of the complete event. Only for relativelyrare kinematical configurations (and/or extra jets from initial state radiation as includedin our simulation), E miss T of the complete event can assume large values. For P2 with m ˜ g − m χ = 200 GeV the jets from the first step ˜ g → q + ¯ q + χ of the decay cascadeare harder, leading to less E miss T . One aim is to study the impact of this difference on thedistributions of kinematical variables.For all benchmark points we assumed practically decoupled squarks with masses of3 TeV; then the gluino pair production cross section from prospino at NLO is 128 fb. (Sincestops and sbottoms are assumed to have masses of 3 TeV as well their pair productiondoes not contribute to the signal.) We deliberately chose identical gluino masses for allpoints in order to maintain a common production cross section; therefore all differences incontributions to signal regions and kinematical distributions originate from the neutralinosector. The masses of the latter are recalled in Table 1 below.In addition we indicate in the Table 1 in how far the benchmark points GMSB, P1and P2 satisfy constraints from the six signal regions of the CMS on- Z searches in [3](including 30% systematic errors from the simulation). The 95% CL upper limits for thesix signal regions of the CMS on- Z searches had already been obtained in [9]. We find thatthe central values of event yields of the benchmark points are below these 95% CL upperGMSB P1 P2Gluino/neutralino masses m ˜ g 800 800 800 m χ (GMSB), m χ (P1, P2) 600 790 600 m ˜ G (GMSB), m χ (P1, P2) 0 690 500Constraining signal regions S CMS, N j ets ≥ 2, 100 < E miss T < 200 207 2.0 ± ± ± N j ets ≥ 2, 200 < E miss T < 300 20 2.6 ± ± ± N j ets ≥ 2, 300 < E miss T ± ± ± N j ets ≥ 3, 100 < E miss T < 200 89 1.9 ± ± ± N j ets ≥ 3, 200 < E miss T < 300 16.1 2.4 ± ± ± N j ets ≥ 3, 300 < E miss T ± ± ± ± ± ± ± ± ± ± ± ± Z SR (obs. excess 18.4) 9.8 ± ± ± Z searches in [3], and in the most constrainingsignal regions CT, EM and ET of the ATLAS search [17]. The ranges of E miss T for the sixCMS signal regions are given in GeV. The last line indicates the contributions to the ATLASon- Z signal region. 5imits with the exception of P2 in the bins N j ets ≥ 2, 200 < E miss T < 300 and N j ets ≥ < E miss T < CL s = CL s + b /CL b values for P2 in these bins are 0.11 and 0.09, respectively, i.e. wellabove the 95% CL exclusion limit of 0.05.Out of the 10 signal regions in the ATLAS search [17] for jets and E miss T we showthe event yields for the signal regions CT, EM and ET which give the largest ratio eventyield/ S for the points P1, P2 and GMSB, respectively. All these signal regions require E miss T > 160 GeV, p T > 130 GeV for the leading jet, and p T > 60 GeV for 3 additional jets(CT), p T > 60 GeV for 5 additional jets (EM and ET). EM and ET differ by m eff ( incl. ) > / E miss T in order to saturate the bound from the signal region ET.Finally we compare the contributions of the benchmark points GMSB, P1 and P2 to theATLAS on- Z signal region, summing dielectrons and dimuons, in the last line of Table 1.We see that a price has to be paid for the very compressed gluino − χ − χ spectrum inP1: Due to the softness of the jets, not enough jets satisfy the cut N jets ≥ 2. The GMSBpoint seems to do quite well, despite its gluino branching fraction being reduced by a factor ∼ / 10. The best fit is given by P2 with its less compressed gluino − χ − χ spectrum.Next we consider the distributions of kinematical variables. As stated above we combinethe ATLAS dielectron and dimuon data (despite the different acceptances) in order toenhance the visibility of possible trends. We only show the (dominant) statistical error ofthe data; we are not in a position to combine the partially correlated systematic errors.In the figures below we show the data with the expected SM background contributionsubtracted, with the aim to expose possible desirable features of signal contributions (see [4]for the error attributed to the expected background).We start with E miss T in Fig. 2 where we compare the data with the expected backgroundsubtracted to the GMSB scenario and with the two heavy-neutralino benchmark pointsP1 and P2. We simulated 500.000 events for each scenario. Each expected event for theLHC run I as shown in Fig. 2 corresponds to 10 simulated events, which allows to estimatethe statistical errors. These are smaller than the estimated systematic errors from oursimulation, and much smaller than the statistical error of the data.The measured event numbers seem to decrease continuously with E miss T (within theerror bars, and note that the rightmost bin includes the overflow), whereas the E miss T distributions of the GMSB and P1 points are nearly flat: In these scenarios, nearly allenergy is transformed into missing energy which prefers accordingly large values of E miss T .(Note that E miss T is shown after the application of all cuts, notably on H T > 600 GeV. ForP1 with its compressed spectrum this cut selects atypical kinematical configurations withparticularly large E miss T .)For a quantitative comparison we compute the reduced χ statistic χ = 1 N bins − N bins X i =1 ( N d − b ( i ) − N S ( i )) σ ( i ) (3.1)6 E v en t s E Tmiss [GeV] GMSBP1P2data--background Figure 2: Comparison of E miss T from the data in [4] (with the expected background sub-tracted) to the benchmark points GMSB, P1 and P2 defined in the text. Error bars on thedata are statistical only. The rightmost bin includes the overflow.for each benchmark point, where N d − b ( i ) is the data with the expected background sub-tracted (as shown in Fig. 2). σ ( i ) combines the statistical error of the data shown in Fig. 2and the systematic error of 30% of our simulation (with respect to which the systematicerror of the background is negligible).We obtained χ = 0 . 69 for GMSB, χ = 0 . 85 for P1 and χ = 0 . 61 for P2. Hencethe scenario P2 with its larger splitting between the gluino and the χ masses describesbest the shape of the E miss T distribution. Of course, the scenario P2 profits also from itslarger total event rate.In Fig. 3 we compare the data on H T (with the expected background subtracted) withthe GMSB scenario and the benchmark points P1 and P2. Since H T represents most of thevisible transverse energy, the point P1 with its compressed spectrum peaks at low valuesof H T . This coincides with the trend of the data, but the total signal rate (limited byconstraints from CMS) is small, as indicated in Table 1.For the reduced χ statistic we find χ = 0 . 54 for GMSB, χ = 0 . 69 for P1 and χ = 0 . 36 for P2. Again, the benchmark point P2 provides the best agreement with theshape of the distribution despite its somewhat less compressed spectrum.Finally we turn to the distribution of the jet multiplicity in Fig. 4. The trend of the datatowards low jet multiplicities is reproduced only by P1 with its excessively low signal rate.The jet multiplicity of simulations is sensitive, amongst others, to the matching betweensoft and hard QCD radiation, accordingly this quantity has to be considered with somereserve.For the reduced χ statistic we find χ = 1 . 03 for GMSB, χ = 1 . 08 for P1 and χ = 1 . 57 for P2. In this case the trend of the data is not well reproduced by the pointP2. But since the scenario P2 provides the best fit to the ATLAS signal rate and the7 E v en t s H T [GeV] GMSBP1P2data--background Figure 3: Comparison of H T from the data in [4] (with the expected background subtracted)to the benchmark points GMSB, P1 and P2 defined in the text. Error bars on the data arestatistical only. The rightmost bin includes the overflow. E v en t s Jet Multiplicity GMSBP1P2data-background Figure 4: Comparison of the jet multiplicity from the data in [4] (with the expected back-ground subtracted) to the benchmark points GMSB, P1 and P2 defined in the text. Errorbars on the data are statistical only. E miss T and H T distributions, it would be interesting to know about SUSY extensions ofthe Standard Model which share the features of this simplified model. As discussed in theintroduction, this is possible within the NMSSM.Using the spectrum generator NMSSMTools [25, 26] with decay branching fractionscomputed by NMSDECAY [27] (based on HDECAY [28]) we found that the following8egion of the parameter space of the Z -invariant NMSSM shares the following propertieswith the point P2: • Heavy (decoupled) squarks in order to satisfy constraints from searches for eventswith jets and E miss T in the presence of a gluino with a mass of 800 GeV. • A bino-like neutralino χ with a mass of 600 GeV, but winos and higgsinos slightlyheavier than the gluino. (The running gaugino masses do not satisfy SU(5)-like rela-tions at the GUT scale.) • A singlet-like neutralino χ with a mass of 500 GeV. Then the branching fraction forthe decay χ → χ + Z is 100%. • The loop-induced gluino two-body decay ˜ g → g + χ should be suppressed, since itwould not contribute to the signal. It is induced by the higgsino component of χ ,and can be of similar order of the desired gluino three-body decay ˜ g → q + ¯ q + χ .The singlino-higgsino mixing is proportional to the NMSSM-specific Yukawa coupling λ [18], and λ should not exceed ∼ . 3. (The loop-induced gluino two-body decay˜ g → g + χ leads to similar signals as ˜ g → q + ¯ q + χ , but it can be expected that itwould improve the jet multiplicity distribution shifting it towards smaller values.)The remaining parameters can be chosen to obtain a Standard Model-like Higgs bosonwith a mass of ∼ 125 GeV. We have checked that a corresponding point in the parameterspace with all squark masses of 2.5 TeV (leading to a gluino pair production cross sectionof ∼ 150 fb in order to compensate for a gluino BR into q + ¯ q + χ slightly below 100%),tan β = 3 . µ eff = 800 GeV, λ ∼ . κ ∼ . A λ ∼ . A κ ∼ − 50 GeV(see [18] for the definitions of the latter parameters) has the properties of P2 and would notbe distinguishable from P2 regarding the different observables shown above in Figs. 2–4. We studied to which extent SUSY extensions of the SM can describe the excess of eventsobserved by ATLAS in the on- Z signal region, respecting constraints by CMS on similarsignal channels as well as constraints from searches for jets and E miss T . For viable scenarioswe compared the distribution of kinematical variables to the data, combining dielectronand dimuon events.Due to hadronic Z -decays, GMSB-like scenarios are typically in conflict with constraintsfrom searches for jets and E miss T . Assuming a 100% branching fraction for the gluino decaycascade including the χ → ˜ G + Z decay, these scenarios become viable only if the gluino hasa mass above ∼ . 05 TeV implying a small contribution ( < χ → ˜ G + Z decay) to ∼ m ˜ g ∼ 800 GeV may contribute significantly to the signal region, remainingwithin the 95% CL limit of CMS. However, the H T and notably the E miss T distributions donot coincide well with the trends of the data. Therefore we studied alternative scenarioswith two massive neutralinos χ , χ . In order to compare the impact of different neutralino9pectra to GMSB-like scenarios and among themselves for fixed gluino pair production crosssections and gluino masses, we fixed the latter also to 800 GeV.A very compressed ˜ g − χ − χ spectrum reproduces somewhat better the trend of the H T distribution, but does not improve the shape of the E miss T distribution. In particular, thecontribution to the ATLAS signal region cannot be enhanced significantly while remainingwithin the 95% CL limit of CMS.A less compressed ˜ g − χ − χ spectrum provides the best fit to the total number of eventsin the ATLAS in the on- Z signal region, as well as to the H T and E miss T distributions; onlythe jet multiplicity is still not well reproduced. (Larger ˜ g − χ mass splittings as assumedhere become again sensitive to constraints from searches for jets and E miss T .) We found thatsuch a scenario can be realised within the NMSSM.A somewhat different approach has recently been persued in [9], where the space ofthe two lightest neutralino masses within the NMSSM was scanned systematically in orderto maximise the contribution to the ATLAS on- Z signal region respecting existing con-straints. The shapes of the kinematical variables have not been studied, however. Still,their main results coincide with ours: Whereas compressed spectra make it easier to satisfyconstraints from other SUSY searches, the contributions to the ATLAS on- Z signal regionare suppressed as well. For gluino masses below ∼ 800 GeV, only a small corner in theplane of the two lightest neutralino masses survives the 95% CL limits of CMS. Within thiscorner (for ˜ g , χ , χ masses of 650, 565 and 465 GeV, respectively) the authors found amaximal contribution of about 11 events to the ATLAS on- Z signal region.Moreover, the authors of [9] considered constraints from signal regions in [14] (2jW and4jW) which are not implemented in the CheckMATE version 1.2.0 [16] used here. Theauthors applied these constraints to our benchmark point P2 and obtained a ratio for theevent yield/ S of 1.19, i.e. about 20% too large, but within the systematic errors from thesimulation. A similar excess holds for this point actually also for two CMS signal regionsconsidered in Table 1. We recall that identical gluino masses of 800 GeV were chosen for allpoints to simplify comparisons. A slightly heavier gluino mass of ∼ 825 GeV would reducethe gluino pair production cross section by ∼ Z signal region would drop to ∼ 11 events. This number coincides with the maximumfound in [9] for the slightly different point above.Clearly, if the excess observed by ATLAS indicates the presence of particles beyond theStandard Model, it should become more visible in both ATLAS and CMS experiments atthe run II of the LHC. But since it is present in the ATLAS analysis of the available datafrom run I we found it appropriate to discuss possible interpretations.Within the class of models considered here, fits to the event numbers and shapes of theATLAS on-Z can be improved with respect to the GMSB scenarios considered in [4]. 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